Functional Nanocomposites Based on Fibrous Clays

EDUARDO RUIZ-HITZKY, MARGARITA DARDER, ANA C. S. ALCÁNTARA, BERND WICKLEIN AND PILAR ARANDAa

1.1 Introduction

Since the beginning of the polymer age, silicates – like clays and other finely particulated solids such as silica, calcium carbonate and carbon black – have been incorporated as fillers at the micrometer dimension into plastics and elastomers with the aim of improving the mechanical and rheological properties of these polymers. Kaolinite was the clay mineral initially most widely used as a silicate filler of diverse polymeric matrices. More recently, swelling clays such as smectites have received great interest due to their ability to exfoliate, giving rise to elemental silicate platelets of 1 nm thickness, which represent a way to develop fillers at the nanometer dimension (nanofillers). In this context, the concept of clay delamination encompassed by its high dispersion in polymers, introduced by Fukushima and other researchers at the Toyota Central Laboratory almost three decades ago, represents a revolutionary idea, not only for the use of clays as reinforcing charges but also to introduce functionality in the resulting materials.

Very quickly, polymer-clay nanocomposites became a popular topic with a rapid increase of publications and registered patents and, in the last 20 years, about 13 000 articles and 200 patents have been published or registered, according to the ISI Web of Science data. Practically all these works make reference to polymer-clay nanocomposites belonging to the smectite layered clays family, i.e. 2:1 charged phyllosilicates, such as montmorillonite, hectorite and saponite natural clay minerals as well as analogous synthetic hectorites and fluoro-hectorites (e.g. LAPONITE[R] and the so-called "synthetic mica"). The smectite layers exhibit a high aspect ratio as each one is approximately 1 nm thick, while the diameter may rise to several microns or larger, as is the case with vermiculites. So, polymer-clay nanocomposites based on smectites have been extensively studied from basic aspects to applications, and a significant number of reviews have been published on this topic (see, for instance, ref. 5–11). Compared to smectites, other types of clay minerals, such as kaolinite, halloysite, imogolite, sepiolite and palygorskite, have barely been studied as nanofillers of polymers. However, as we report below in this chapter, the structural and textural characteristics of the fibrous clays could be of great benefit for the properties and applications of the novel polymer–clay nanocomposites derived from them.

The sepiolite and palygorskite fibrous clays also appear to be attractive nanofillers to reinforce polymer matrices. They do not exhibit intercalation capacity, but these two natural silicates offer interesting characteristics, such as microporosity and large specific surface area. Interestingly, the presence of hydroxyls (silanol groups, Si–OH) at their external surface allows easy functionalization by controlled modification based on chemical reactions, e.g. with coupling agents or by anchoring of nanoparticles (NPs). This behavior allows the preparation of a wide variety of polymer–clay nanocomposites provided with diverse functionality.

Sepiolite is a natural hydrated magnesium silicate with fibrous morphology, displaying a crystal structure consisting of talc-like ribbons arranged parallel to the fiber direction (c-axis) with Si12O30 (OH)4(H2O)24x8H2O as the ideal formula (Figure 1.1A). To a variable extent, isomorphous substitution of magnesium at the octahedral layer by trivalent metals, mainly Al(iii), provokes a charge deficiency in the structure that is compensated by extra-framework cations. This is the origin of the cation-exchange capacity (CEC) of sepiolite samples, which has been generally established in the 10–20 mEq 100 g-1 range, depending on the origin of the clay and always being about 4–5 times inferior to typical CEC values found in smectite clays. As often occurs in clays formed in lacustrine sediments, certain sepiolite samples show that hydroxyls located at the octahedral layers (mainly Mg–OH) can be partially substituted by fluorine. One of the most interesting features of this silicate is the existence of microporosity ascribed, as occurring in zeolites, to their backbone structural arrangement – in this case with discontinuity of the phyllosilicate layers. This characteristic results in an alternating distribution of structural blocks, each one composed of two tetrahedral silica sheets and a central sheet of magnesium hydroxide, which determine the presence of structural cavities (tunnels) that grow along the c-axis direction (Figure 1.1A). The cross-section of the sepiolite tunnels is 1.6 × 0.37 nm2, which determines a structural microporosity typically superior to 0.3 cm3 g-1 as determined from nitrogen adsorption isotherms. Palygorskite (Figure 1.1B) is a related silicate with a higher content of...